Ventricular tachyarrhythmias and atrial fibrillation occurring in patients with structurally abnormal hearts are the most important arrhythmias in contemporary cardiology, and despite much progress, remain therapeutic challenges. Invasive electrical studies of the heart (electrophysiologic studies) are often used in the diagnosis and therapy of arrhythmias, and many arrhythmias can be cured by selective destruction of critical electrical pathways with radiofrequency (RF) catheter ablation. A major limitation in studying arrhythmias in patients, however, is the lack of ability to accurately correlate anatomical and electrical information. Another major limitation is the lack of ability to visualize ablated areas of myocardium during catheter ablation procedures, making it difficult to confirm the presence of ablated lesions in the desired locations. We are developing ways of combining the anatomic information from magnetic resonance imaging (MRI), with electrophysiologic testing and ablation. We hypothesize that MRI, with MRI-compatible (non-magnetic) electrode catheters, catheter-tip location sensors, intracardiac receivers, real-time MRI scanner control, remote-control catheter manipulators, and 3- dimensional imaging software can (1) provide the ability to accurately visualize cardiac anatomy, (2) provide accurate navigation of catheters without radiation, (3) provide the ability to visualize ablated lesions, and (4) aid in producing more accurate electrical maps. Our previous project dealt with (1) technology development, (2) demonstration of the feasibility of MRI guidance of catheters in animals, and (3) lesion visualization in animals, and in patients with atrial arrhythmias. This competing continuation deals with (1) additional technology development, (2) improved integration of the different subsystems, (3) study of the determinants of successful ablation in patients undergoing standard ablations, and (4) broadening of the applications to real-time MRI guided therapy in patients with atrial and ventricular arrhythmias. The technologies developed in this project, should, in addition, be applicable to using MRI to guide interventional procedures in general. This project is a partnership between the Johns Hopkins University School of Medicine (Medicine, Radiology, and Biomedical Engineering), Robin Medical Inc., MicroHelix Inc., NaviCath Inc., and Irvine Biomedical, Inc. All entities have supplied resources to the project, and will continue to share the costs of the project. The School of Medicine has an ongoing commitment to developing cardiac MRI, as demonstrated by its substantial investment in MRI scanners, including one adjacent to the cardiac catheterization laboratory. These scanners have a magnet that is short enough to allow access to the groin vessels for placement of catheters for diagnostic and interventional procedures. Robin Medical has developed technologies for precisely localizing the tip of a catheter inside an MRI scanner, and is developing technology for deflecting the tip using the MRI magnetic fields. MicroHelix is developing specialized catheter electrodes that reject MRI electromagnetic interference. NaviCath is developing an MRI-compatible system for remote manipulation of catheters that will allow catheters to be manipulated in patients in MR scanners that are too long to allow easy access to the groin vessels. Irvine Biomedical is supplying non-magnetic electrode catheters for use in the MRI scanner.